The present invention relates to isolators and, in particular, to protection circuits for isolators that guard against common mode transients.
Isolators are devices that exchange data signals between two galvanically isolated circuit systems. The circuit systems each operate in different voltage domains, which may include different source potentials and different grounds. Isolation devices may provide data exchange across an isolation barrier, which maintains the galvanic isolation. Typical isolation devices include micro-transformers, capacitors, and magneto-resistors.
Many isolator devices are differentially-driven. That is, signal content (“VSIG”) is represented in a pair of signals that deviate differentially about a common mode voltage (“VCM”). A first signal may deviate from the common mode voltage VCM by an amount VSIG (e.g., V1=VCM+VSIG) and a second signal may deviate from the common mode voltage VCM by the same amount ΔV but in complementary fashion (e.g., V2=VCM−VSIG). In this example, the VSIG value represents signal content. Isolator circuitry often is designed using the common mode voltage VCM as a design factor to transmit and/or receive these differentially-driven signals representing signal content.
Isolator devices often are used in noisy environments. They may be subject to electro-magnetic transients that cause signal corruption in the signals that are transmitted and received by such systems. Some transients cause deviation in the common mode of the signals being transmitted by the system. Thus, where a differential signal pair ideally would deviate from the common mode in differential fashion, a common mode transient may cause the differential signals to vary together in a manner that interferes with operation of the isolator (e.g., V1=VCM+VEMI+VSIG, V2=VCM+VEMI−VSIG). Some transients may cause these signals to exceed the supply voltages (VDD or ground) of the circuitry that receive and decode the signals, which can lead to the signals being decoded incorrectly.
The inventors perceive a need in the art for an isolator system that protects against common mode transients in operation.